What if you knit across those 13 stitches and then put them on the holder. The yarn would be in the right place to cast on the 3. I'd do a backward loop cast on on the right needle and then finish the round.
Does that make sense with what you've got in front of you?
OR, since the 13 are supposed to be the thumb, then you could knit the 2, put the 13 on a piece of yarn, cast on the three and bringing the cast on stitches right next to the 2 that you knit. This would make the thumb stitches close in a circle. Is that clear in any way?
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry